Latex paints are water-borne systems that are generally manufactured by a two-step process. First, a dispersion phase, commonly referred to as the grind phase, is prepared by mixing dry paint pigments with other grind phase components, including most other solid powder formulation materials, under constant high shear agitation to provide a high viscosity and high solids mixture. This part of the process is designed to effectively wet and dis-agglomerate the dry pigments and stabilize them in an aqueous dispersion.
The second step of the paint manufacturing process is commonly referred to as the letdown or thindown phase, because the viscous grind is diluted with the remaining formulation components, which are generally less viscous than the grind mix. Typically, the latex resins, any predispersed pigments, and any other paint materials that only require mixing and perhaps moderate shear, are incorporated during the letdown phase. The letdown phase may be done either by sequentially adding the letdown components into a vessel containing the grind mix, or by adding the grind mix into a vessel containing a premix of the latex resins and other letdown components, followed by sequential addition of the final letdown components. In either case, constant agitation is needed, although application of high shear is not required.
Biocides are typically included in latex paints to provide the paint with resistance to microorganisms. Biocides may be incorporated at different stages of the paint manufacture process, however they are commonly added in the last steps to decrease their exposure to high temperature or potential deactivating reagents.
Several water-insoluble biocides, such as BIT (1,2-benzisothiazol-3(2H)-one), are known to be highly effective at controlling organism growth in aqueous media. However, incorporation of water insoluble biocides in latex paint is challenging because of the potential of the water insoluble material to agglomerate and thus result in paint with unacceptable properties. As a result, incorporation of water insoluble biocides has required that they be delivered as a low-solids solution or dispersion in a solvent, such as a glycol, or as dispersion in water.
The use of solvents for delivering water-insoluble biocides into paints is, however, undesirable for several reasons. Solvents increase the presence of volatile organic compounds (VOCs) both during paint manufacture and in the final paint product. Solvents also add to the material cost of the paint, increase inventory and storage needs, and increase transportation costs. As a consequence, a significant advance in the field, from both a cost and environmental impact perspective, would be the provision of new techniques for incorporating water insoluble biocides into latex paints with limited use of solvents.